The present invention relates to a numerical control unit which adopts what is called an F1-digit feed command system, which is capable of driving a motor at a feed speed designated by a feed speed-designation dial on an NC control panel.
The F1-digit feed command system permits the operator to carry out actual machining while controlling the feed speed for each process by turning a feed speed-designation dial on a control panel installed on the side of an NC cabinet or a machine, and this is one of the feed speed command systems suitable for use when machining dies or the like if the optimum cutting speed is unknown before actual machining is started. In a conventional numerical control unit employing this kind of system, a voltage-controlled variable frequency oscillator is driven by a DC voltage proportional to the amount that a knob of the feed speed-designation dial is turned to generate a signal of a frequency proportional to the amount of the rotational movement of the knob, and the frequency is counted by a main processor section and then the motor is driven at a feed speed proportional to the results of the counting. The prior art unit thus includes an analog oscillation circuit, and hence it has the defects that it has to be adjusted at many places and is complicated in circuit construction because of the necessity for reducing variations in the oscillation frequency.
The present invention offers a solution to such defects of the prior art and has for its object to provide a numerical control unit which achieves the F1-digit feed command system with a simple circuit structure.